It is well known that the combustion of fossil fuels, e.g., gasoline, generates deleterious automobile exhaust containing carbon monoxide, carbon dioxide, oxides of nitrogen (primarily NO.sub.x), water, and nitrogen. The exhaust also can contain a wide variety of hydrocarbons and also particulates including carbon and oxidized carbon compounds, metal oxides, oil additives, fuel additives, and breakdown products of the exhaust system, including the exhaust-control catalysts.
These exhaust products can combine in a large variety of ways in the atmosphere, particularly since the amounts of each material change with operating conditions and the mechanical state of the vehicle. The photochemical reaction between oxides of nitrogen (NO.sub.x) and hydrocarbons (HC) that caused the original interest in the automobile as a source of pollution has been investigated extensively.
Due to the now well-appreciated harmful effects of the vehicle emission pollutants to both health and to the environment in general, ever increasing stringent air quality standards are being imposed on emissions at both a federal and state level.
Also, many commercial operations, industrial processes or even home heating systems generate noxious gaseous chemical by-products, the removal of which must comply with federal or state regulations. These regulations may be highly expensive to meet with, if not cost prohibitive, using current exhaust gas treatment technology. Therefore, the anticipated benefits of improved environmental quality confers a very high value on any new engineering technology that might be useful to meet the regulatory air quality standards.
A known technology for control of exhaust gas pollutants from both stationary and mobile sources is their catalyzed conversion into more innocuous chemical species. Conventional oxidation catalysts used in this regard promote further burning of hydrocarbons and carbon monoxide in the exhaust gas. The normal operating temperature is 480.degree. to 650.degree. C. Oxidation catalysts in current use normally start oxidizing within two minutes after the start of a cold engine and will operate only when the catalytic species is sufficiently heated to achieve an activation temperature.
Known oxidation catalysts consist of platinum and mixtures of platinum and other noble metals, notably palladium. These metals are deposited on alumina of high surface area. The alumina ceramic material is typically capable of withstanding very high temperatures. The ceramic core has thousands of passages--about 240 per square inch. These passages present an enormous surface area for contact with the exhaust as it passes through the catalytic converter. The ceramic passages are coated with the platinum and palladium metals. These metals provide the catalysts.
When properly contained in the muffler-like shell of the catalytic converter, the catalysts will reduce hydrocarbon and carbon monoxide pollutants by changing them into more harmless products of water vapor and carbon dioxide. Another common form of oxidization catalyst involves a monolith in a honeycomb configuration to provide the necessary surface area and a top layer of the deposited catalytic metal species. The selection of one or the other above catalytic configurations is dictated by the kind of vehicle usage, as understood in the field.
However, conventional catalytic devices and catalytic species used therein have serious drawbacks in that they typically are susceptible to poisoning, i.e., deactivation resulting from chemical changes caused by the combined effects of thermal conditions and contamination as characterized by a chemical reaction of a contaminant with the supported catalysts. For instance, the most notorious poison for vehicular catalytic converters is the lead compound used as an anti-knocking agent. The poisoning of the catalysts by the contaminant, such as lead, is irreversible.
Moreover, many conventional catalysts also are susceptible to inhibition, or so-called reversible poisoning because of its temporary effect, due to exposure of the catalytic species to many common exhaust gas components such as carbon monoxide, nitrogen oxides or even some reduced sulfur compounds.
Compounding the poisoning problem encountered with many conventional catalysts used in treatment of exhaust gases is the demand for a more versatile catalytic species having applicability to diverse areas of exhaust gas treatment.
For instance, the federal and state regulatory attitude is ever increasingly stricter in imposing emission control standards covering a plethora of both commercial and private emission sources, e.g., coal burning plants and stoves, wood burning stoves, garbage incineration, used tire incineration, and not merely vehicle exhaust regulation.
Therefore, in an effort to meet current and perhaps even stricter future environmental air quality objectives, many public and private concerns have urgently awaited any possible innovations in the catalytic exhaust control field which might meet these standards.